Our long-term goal is to delineate the molecular mechanisms underlying the maintenance of endoplasmic reticulum (ER) homeostasis by two key quality-control systems in the cell, ER-associated degradation (ERAD) and unfolded protein response (UPR). Recently, we have identified a novel regulator of IRE1?, the most conserved sensor of the UPR (He et al. Dev Cell 2012), and reported the generation and characterization of inducible Sel1L knockout (Sel1LIKO) mouse and cell models (Sun et al. PNAS 2014), a cofactor of the ubiquitin ligase Hrd1 in mammalian ERAD. In the preliminary data of this application, we discovered a unique crosstalk between UPR and ERAD, namely the regulation of IRE1? stability by the Sel1L-Hrd1 ERAD complex. Here we showed that loss of Sel1L-Hrd1 ERAD function leads to a dramatic accumulation of IRE1?protein in various tissues and cell types including pancreas, colon, spleen, adipose tissue, MEFs, macrophages and etc. IRE1? accumulation in the absence of Sel1L is independent of transcriptional regulation, pointing to a post-transcriptional control. Indeed, IRE1? interacts with Sel1L and is significantly stabilized in the absence of Sel1L or Hrd1. Thus, these data point to IRE1? as a misfolding- prone Sel1L-Hrd1 ERAD substrate. Here we propose to test the hypotheses that IRE1? is an ERAD substrate and that the Sel1L-Hrd1 ERAD complex negatively regulates the amplitude of IRE1? signaling by mediating its degradation. Taking advantage of systems and tools that we have generated for both Sel1L ERAD and IRE1?, we will accomplish the following Aims: (1) Determine the biological significance of IRE1? ERAD on IRE1? signaling and cell survival; (2) Determine how misfolded IRE1? protein is recognized and delivered to the Sel1L-Hrd1 ERAD complex; and (3) Elucidate how misfolded IRE1? protein is degraded by the Sel1L-Hrd1 ERAD complex. Successful completion of this study may not only provide key insights into IRE1? and ERAD biology, but also uncover a novel regulatory mechanism for IRE1? signaling. This study will provide an unprecedented opportunity to investigate the complicated mechanism of ERAD using an endogenous substrate with great physiological significance, thus exerting a powerful influence on our views of physiological ERAD and UPR biology.